﻿Vol. 66.~] CLASSIFICATION OF IGNEOUS BOOKS. 491 



varying conditions. If we perfectly understood the influence of 

 conditions on texture we might sketch the history of each rock 

 during its consolidation period. But chemical composition has 

 also a general and very important bearing upon. texture, in that the 

 influence of given conditions varies with the chemical character 

 of the magma. Ko set of conditions produces the same result on all 

 magmas. 



A discussion of the origin of various textures is not necessary 

 for our present purpose. It will suffice to recall that we now 

 know rate of cooling to be the principal factor in the production of 

 texture, .with varying results according to the composition of the 

 magma. Rate of cooling depends on initial temperature of magma, 

 mass, pressure, temperature, and conductivity of wall- rock, escape 

 of gases from solution, and on other factors. Another factor of 

 great influence in producing texture is movement of the partly 

 solidified magma. Wherever this occurs orientation of mineral 

 particles is a natural result. 



Applying these considerations to the granular fabric, it is plain 

 that slow cooling and uninterrupted crystallization may most 

 commonly be expected for deep-seated masses ; but, wherever the 

 necessary conditions are realized, the granular fabric must result. 

 Granite, once believed to be possible only as a result of crystal- 

 lization at great depth, is now known to have formed also very 

 near the surface. Dykes which penetrate a volcanic centre where 

 long eruptions have been in progress have often the same texture 

 as that exhibited by plutonic masses of the same rock. Basic 

 magmas find conditions favourable to granular fabric provided in the 

 interior of surface- flows, as well as in intrusive plutonic bodies. 1 



Use of texture in classification. — Texture has always 

 been very prominent in the classification of igneous rocks. Al- 

 though its systematic application has seldom been consistent, we 

 have, in fact, one set of names for what are stated or assumed to 

 be granular rocks, and another set for porphyritic, rluidal, and 

 more or less glassy rocks, which are their chemical equivalents. A 

 general dependence of texture on conditions of consolidation was 

 long ago recognized, and the convenient assumption was made that 

 pressure is the principal element of the conditions favourable to 

 the production of the granular texture. Further, pressure was 

 considered a function of depth. The latter is true only if tan- 

 gential stress and eruptive force bearing on an intrusive magma 

 are ignored; and, as above mentioned, pressure is now known 

 to be less potent in determining texture than a variety of other 

 conditions. 



The old idea of the relation of pressure to production of the 



1 This was recognized by B. von Cotta, who pointed out that, if the chlorite 

 of diabase should prove to be of secondary origin, ' the whole original 

 difference would perhaps lie in the level at which solidification took place' 

 « Gesteinslehre ' 2nd ed. (1862) p. 84. 



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